Scrubbing fluid and methods for using same

ABSTRACT

A scrubbing solution for removing contaminants, including particularly hydrogen sulfide, from a fluid. The scrubbing solution includes at least one scrubbing reagent which has a primary or secondary amine and an acid, which may be phosphoric acid. The fluid being scrubbed is passed through the scrubbing solution. The contaminants react with the scrubbing reagent securing them in the scrubbing solution. The fluid being scrubbed and the scrubbing solution are then separated. The scrubbing solution is heated and, if the scrubbing solution is under pressure, the pressure is reduced. The acid facilitates thorough removal of the contaminants, and especially the hydrogen sulfide, from the scrubbing solution. The scrubbing solution is then ready for reuse. Because the scrubbing solution is rendered substantially free of hydrogen sulfides, it can absorb other sulfide contaminants that might not otherwise be absorbed.

FIELD OF THE INVENTION

The invention relates to removing contaminants from fluid streams ingeneral and gas streams in particular.

PRIOR ART

Scrubbing solutions are known. Commonly, the scrubbing solution willinclude one or more reagents selected to react with the contaminants tobe removed from a fluid. The scrubbing solution is usually a liquid, inwhich the fluid to be scrubbed is preferably no more than minimallysoluble under ambient conditions and less so under elevatedtemperatures. When the fluid to be scrubbed is a gas, the gas will bepassed through the scrubbing solution, and the reagent(s) will reactwith the contaminant(s). The gas will exit the scrubbing solution whilethe contaminants will remain in the scrubbing solution, bound to thereagent(s). When the fluid to be scrubbed is a liquid, the scrubbingsolution will be mixed with the liquid and the reagent(s) will reactwith the contaminants. The liquids will be separated according to avariety of conventional methods, including gravity/density separation.Again, the contaminants will remain in the scrubbing solution.

The scrubbing solution is typically used in a cyclical process in whichthe solution is recycled. To efficiently remove the contaminants duringabsorption, the contaminants captured by the scrubbing solution must beremoved from the scrubbing solution before it is reused.

While variance of temperature and pressure has long been used to removecontaminants from scrubbing solutions, the release of contaminants viachanges in pressure and temperature alone tends to be less thancomplete. Additives may be introduced to the scrubbing solution tofacilitate the release of the contaminants during desorption. However,the use of additives presents a fundamental design obstacle to thesystem. The reagents need to readily react with the contaminants, yetgive up the contaminants easily. Additives which enhance the ability ofreagents to release the contaminants tend to inhibit the ability of thereagents to capture the contaminants and vice versa. Accordingly, ascrubbing solution meeting the following objectives is desired.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a scrubbing solution thatcan effectively remove contaminants from a target fluid.

It is another object of the invention to provide a scrubbing solutionthat can efficiently release contaminants after removing them from atarget fluid.

It is still another object of the invention to provide a scrubbingsolution that can efficiently release contaminants without substantiallyinhibiting the ability of the scrubbing solution to capturecontaminants.

It is yet another object of the invention to provide a scrubbingsolution configured to remove contaminants containing sulfide from atarget fluid.

It is still another object of the invention to provide a scrubbingsolution configured to remove contaminants containing mercaptans from atarget fluid.

It is yet another object of the invention to provide a scrubbingsolution configured to remove contaminants containing hydrogen sulfidefrom a target fluid.

It is still another object of the invention to provide a scrubbingsolution configured to remove contaminants containing carbonyl sulfidefrom a target fluid.

It is yet another object of the invention to provide a scrubbingsolution configured to remove contaminants containing carbon dioxidefrom a target fluid.

SUMMARY OF THE INVENTION

The invention comprises a scrubbing fluid and a method of using thesame. The scrubbing fluid contains one or more amine containingscrubbing reagents. Examples of amine containing scrubbing reagentsinclude methyl diethanolamine (“MDEA”) and piperazine. At least one ofthe scrubbing reagents will preferably contain a primary or secondaryamine. During absorption, the scrubbing solution is applied to a fluidto be scrubbed. The amines are selected to remove acidic contaminantsfrom the fluid being scrubbed. Common acidic contaminants includehydrogen sulfide (H₂S), carbon dioxide (CO₂), and mercaptans (R—SH). Theamines will react with the contaminants, securing the contaminants inthe scrubbing fluid and removing them from the fluid being scrubbed.

During desorption, the contaminants are removed from the scrubbing fluidso that it can be reused. When the scrubbing fluid is under pressure,the pressure may be reduced during desorption to release some of thecontaminants. Regardless of the pressure of the scrubbing fluid, thescrubbing fluid may be heated to release some of the contaminants.

The scrubbing fluid is also provided with an acid. The inventors havediscovered that the addition of an acid to the scrubbing solutionenhances the ability of the scrubbing fluid to remove hydrogen sulfidewithout compromising the ability of the scrubbing fluid to remove carbondioxide. The enhanced removal of hydrogen sulfide allows the scrubbingfluid to remove additional sulfide contaminants, such as mercaptans.

Preferred acid additives to the scrubbing fluid are polyprotic. Suitableacids include phosphoric acids and sulfuric acids. The phrase“phosphoric acids” is intended to encompass orthophosphoric acid (H₃PO₄)and the oligophosphoric acids: pyrophosphoric acid (H₄P₂O₇);tripolyphosphoric acid (H₅P₃O₁₀); tetrapolyphosphoric acid (H₆P₄O₉); andso forth. The phrase “sulfuric acids” is intended to encompass sulfuricacid (H₂SO₄); persulfuric acid (H₂SO₅); pyrosulfuric acid (H₂S₂O₇);dithionous acid (H₂S₂O₄); tetrathionic acid (H₂S₄O₄); and thiosulfurousacid (H₂S₂O₂). The amount of acid used will depend on the amount ofamine scrubbing reagents in the scrubbing fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of the data from example 1.

FIG. 2 is a chart of the data from example 2.

FIG. 3 is a chart of the data from example 3.

FIG. 4 is a chart of the data from example 4.

FIG. 5A is a chart of the data from example 5.

FIG. 5B is a chart continuing the data from example 5.

FIG. 5C is a chart continuing the data from example 5.

DETAILED DISCLOSURE OF THE BEST MODE

A scrubbing fluid is disclosed. In one embodiment, the scrubbing fluidis an aqueous solution. One or more reagents are added to the aqueoussolution. The reagents are selected to react with the contaminants inthe target fluid. In one embodiment, the reagents are amines. In oneembodiment, one of the reagents is methyl diethanolamine (“MDEA”). Inanother embodiment, one of the reagents is a primary or secondary amine.In another embodiment, one of the reagents is piperazine. In anotherembodiment, MDEA and piperazine are used together.

Other suitable scrubbing reagents include dimethylethanolamine,triethanolamine, monoethanolamine, monomethylethanolamine,diethanolamine, methylpiperazine, morpholine, 2-amino-1-butanol,2-amino-propanol, 2-amino-3-methyl-1-butanol, 2-amino-1-pentanol,2-amino-1-hexanol, 2-amino-1-octanol, aminoethoxyethanol,2-amino-2-methyl-1-propanol, 1-(2-hydroxyethyl)piperazine,diisopropanolamine, and sterically hindered amines such as2-(N-methylamino)-2-methylpropan-1-ol,2-(N-sec-butylamino)-2-methylpropan-1-ol (SBAE), and(2-(N-t-butylamino)-2-methylpropan-1-ol, and Flexsorb SE™, a stericallyhindered alkanolamine (C₈H₁₉NO₂) available from the Monument ChemicalCompany of Pasadena, Tex. The scrubbing solution may be comprised of oneor more of any of the foregoing scrubbing agents or similar aminecontaining scrubbing agents used alone or in combination with other,non-amine scrubbing agents.

In one embodiment, MDEA comprises between about 20 and about 60 percentby weight of the scrubbing fluid and piperazine comprises between about0.1 and about 10 percent by weight of the scrubbing fluid (unlessotherwise indicated, when used herein the term “about” means plus orminus 10 percent). In one embodiment, MDEA comprises about 42.0 percentby weight of the scrubbing fluid and the piperazine comprises about 8.0percent by weight of the scrubbing fluid, with the balance being waterand acid as discussed below. In another embodiment MDEA comprises about35.0 percent by weight of the scrubbing fluid and the piperazinecomprises about 5.0 percent by weight of the scrubbing fluid, with thebalance being water and acid.

In many of the embodiments discussed, the only ingredients in thescrubbing solution were water, acid, and scrubbing reagents. However, ifthe fluid being treated has foaming issues, an anti-foam agent may beadded. Polyglycol and silicone based anti-foam agents will be suitablein most instances. Other potential additives include physical solvents,corrosion inhibitors, and oxygen scavengers.

Some of the common target contaminants are sulfide containing compoundssuch as hydrogen sulfide, mercaptans, and carbonyl sulfide (COS).Another common contaminant is carbon dioxide. Several other targetcontaminants are listed in the figures as discussed in the examples.

Hydrogen sulfide and carbon dioxide are the most common targetcontaminants. However, their presence can make the removal of othertarget contaminants very difficult. For example, the pKa of H₂S is much,much lower than the pKa of most mercaptans at ambient temperatures. Aslong as there is any H₂S in the fluid being scrubbed, the aminescrubbing reagents will react with the H₂S over the mercaptans.Essentially all of the H₂S has to be removed before the amine scrubbingreagents will capture mercaptan contaminants.

Piperazine and MDEA each contain amine functional groups. MDEA is atertiary amine. Piperazine includes two secondary amines. The amines inthese reagents are believed to react well with most, if not all of theabove listed contaminants. In any event, MDEA and piperazines are goodscrubbing reagents for the targeted contaminants, and they workparticularly well in tandem.

In one embodiment of the invention, pressure varies significantlybetween the absorption and desorption sides of the scrubbing process. Onthe absorption side of the process, the pressure may be elevated whilethe temperature ranges from ambient to as high as 180 to 190 degrees F.These conditions facilitate absorption of the contaminants by thescrubbing solution. Specifically, the elevated pressure—typically, atleast about 50 to about 150 psig and often about 1000 psig orhigher-forces the contaminants into solution in the scrubbing solution.The solubility of most gases is inversely related to temperature, somore gas may be dissolved in the scrubbing solution at lowertemperatures. Once in solution, the contaminants may react with andbecome bound to the reagents.

On the desorption side, the pressure will be much lower—typically 10-12psig while the scrubbing solution will be heated above about 190 degreesF. and to as high as about 325 degrees F. and, in one embodiment, toabout 260 degrees F. This tends to cause the scrubbing solution torelease the contaminants absorbed on the absorption side of the process.Many of the contaminants simply dissolved in the scrubbing solution willdegas because of the drop in pressure. Likewise, heating the scrubbingsolution will have a similar effect on gasses dissolved in the scrubbingsolution as the solubility of most relevant gasses goes down astemperature goes up.

Removing dissolved gasses from the scrubbing solution will effect theequilibrium of gasses bound to the scrubbing reagents. The quantity ofcontaminants that will react with the scrubbing reagents depends, inpart, on the amount of contaminants dissolved in the solution. Morecontaminants dissolved in the solution means more contaminants willreact with the reagents. As the quantity of contaminants dissolved inthe scrubbing solution falls, the equilibrium shifts and fewercontaminants will remain bound to the reagents.

The invention involves the addition of an acid to the scrubbing fluid.In one embodiment the acid is phosphoric acid. In another embodiment,the acid is sulphuric acid. The amount of acid used will depend upon theamount of scrubbing reagent present. Where the scrubbing reagents areamines or include amine functional groups, between about 0.004 moles toabout 0.80 moles acid will be provided per mole of amine containingreagent. Where the scrubbing reagents are MDEA and piperazine and theacid, the preferred range of phosphoric acid is between 0.004 to about0.16 moles acid per mole of amine. Where the scrubbing reagents are MDEAand piperazine, the preferred range of sulfuric acid is between about0.004 to about 0.49 moles acid per mole amine. In one embodiment wherethe scrubbing reagents are MDEA and piperazine and the acid isphosphoric acid, between about 0.018 moles to about 0.036 moles of acidare provided per mole of amine containing reagent.

The addition of phosphoric acid to solutions containing tertiary amines,such as MDEA, in order to reduce H₂S levels in the scrubbing solution isknown. However, it is contraindicated in scrubbing solutions thatcontain CO₂ and more than trivial amounts of primary or secondaryamines. Secondary amines, such as piperazines, react with CO₂ to formcarbamates and protonated amines. The prior art teaches that thepresence of CO₂ in the fluid being scrubbed and a primary or secondaryamine in the scrubbing solution nullifies the ability of acid to removeH₂S from the scrubbing solution.

The present invention achieves substantial elimination of H₂S from thescrubbing solution via the addition of an acid, despite the presence ofhigh CO₂ levels and the use of a secondary amine to react with the CO₂.The invention achieves the elimination of H₂S and CO₂ from the scrubbingsolution. The inventors believe that the very low H₂S levels in thescrubbing solution upon its return to the absorption side of theprocess, allows the scrubbing solution to capture substantially all ofthe H₂S from the fluid being scrubbed. This is believed to allow thescrubbing reagents to capture other sulfide containing contaminants,including especially mercaptans, as illustrated in the examples.

During absorption—when the scrubbing fluid is interacting with the fluidto be scrubbed—the temperature will typically be below about 190 degreesFahrenheit, and commonly between about 70 degrees and 185 degrees F.During absorption, the fluids typically move in opposite directions in acolumn, See, e.g., U.S. Pat. No. 10,646,818 which is hereby incorporatedby reference in its entirety. The scrubbing solution typically enters atthe top of the absorption column and moves down while the fluid beingscrubbed enters at the bottom and moves up. The absorption process isexothermic, so the scrubbing fluid heats up as it absorbs contaminants.That means that the scrubbing solution is usually coolest when it entersthe absorption column and hottest near the bottom of the column.Additionally, the contaminants in the fluid are progressively removed asthe fluid moves up the column. The net result of this process is thatthe fluid being scrubbed will be cleanest at the top of the column whereit will also encounter the cleanest and coolest scrubbing solution.Where the fluid being scrubbed is a gas, the partial pressure of thecontaminants in the gas will be lowest at the top of the column.

At the top of the absorption side of the process, the temperature willcommonly be about 120 to 130 degrees Fahrenheit, and warms as itproceeds through the absorption side of the process. Duringdesorption—when the contaminants are being removed—the temperature israised to above about 190 degrees Fahrenheit. When MDEA, piperazines,and other amines are used as the scrubbing reagents, temperatureincreases above about 325 degrees F. are undesirable because they canresult in thermal degradation of the amines. The hottest place on thedesorption side of the process will be where the scrubbing solutionexits the desorption side. This will usually be the boiling point of theamine scrubbing reagents at the pressure being applied to the desorptionside. The pressure will vary depending on the scrubbing reagents and thecontaminants, but temperatures of about 250 to 260 degrees Fahrenheitare typical.

While the pressure drop and temperature increase discussed above willremove many of the contaminants from the scrubbing fluid, the acidfacilitates polishing, allowing for the removal the residualcontaminants from the amines. Once released by the amines, thecontaminants may be removed from the scrubbing solution. Upon return ofthe scrubbing solution to the absorption phase of the process, thetemperature is lowered.

While the process has been described in the context of a significantpressure variation, it will be appreciated that the effect of the acidis not dependant on the pressure change. Many scrubbing processesinvolve treatment of low pressure fluid streams in which there is nohigh pressure to release. Contaminants in such low pressure fluid may betreated with a scrubbing solution in which amine containing scrubbingreagents react with and capture the contaminants. Heating the solutionwill have the same effect on dissolved gasses in the solution discussedabove, and the acid will have the same polishing effect. Once thecontaminants have been removed, the temperature may be lowered and thescrubbing solution is ready for reuse.

EXAMPLES Example 1

FIG. 1 provides comparative results of a scrubbing system operated twodays apart. On day one, the scrubbing fluid consisted of a combinationof MDEA (39.9% by weight) and piperazine (8.1% by weight) and thebalance water. Two days later, the scrubbing fluid consisted of acombination of MDEA (41.3% by weight), piperazine (7.7% by weight) and1.39 percent by weight H₃PO₄. Again, the balance was water. Thiscorresponds to 0.033 moles H₃PO₄ per mole of amine containing scrubbingreagent. The temperature and pressure of the incoming gas streams weresimilar. The amount of contaminants (CO₂ and H₂S) in the gas to bescrubbed were slightly higher on the second day. The flow rate of thescrubbing fluids, provided in millions of standard cubic feet per day(MMSCFD) were quite similar on both days as was the incoming temperatureof the scrubbing fluid. There were slightly more moles of contaminantper mole of amine containing scrubbing reagents on the second day thanon the first. The temperatures in the desorption column were similar.Overhead temperatures are reported. This is the temperature of the steamused to heat the column at the top of the column. Temperatures of thescrubbing fluid within the column were measured using infrared sampling.They reached a maximum of about 150 degrees F. on day 2.

The results for H₂S and CO₂ scrubbing were outstanding. The acidifiedscrubbing solution removed an order of magnitude more H₂S: 0.07 ppm H₂Sremaining in the gas scrubbed with the acid vs. 0.321 H₂S remaining inthe gas scrubbed without the acid. This is believed to be attributableto the removal of more of the H₂S contaminants from the scrubbing fluidduring desorption, the cleaner scrubbing fluid ultimately being able toremove more contaminants on reuse.

Example 2

FIG. 2 provides a comparison of the scrubbing fluids used in a gasscrubbing system. The last four measurements were taken over a six dayperiod. The first was taken a little over a month earlier. All of thescrubbing reagents contained amines and there were no other aminesources in the scrubbing fluid. Thus, the amine measurements reflect thescrubbing reagents in the scrubbing fluid. The gas chromatographymeasurements indicate the total amines present. The alkalinitymeasurement reflects the amines available for reaction with an acid. Theamine scrubbing reagents are basic, and the amount present can bedetermined by measuring how much acid is required to lower the pH tozero. The difference between the gas chromatography numbers and thealkalinity numbers is indicative of the amount of acid contaminants theamine scrubbing agents have picked up during the gas scrubbing process.The heat stable amine salts are indicative of the salts the aminescrubbing reagents have formed during scrubbing with gas contents otherthan sulfides or carbon dioxide.

The CO₂ results reveal an order of magnitude improvement in the acidtreated scrubbing fluid compared to the non-acid treated scrubbingfluid. The H₂S results were even better. Two full orders of magnitudeimprovement were observed. These results indicate that after desorption,the scrubbing fluid treated with the acid contains substantially fewercontaminants compared to scrubbing fluid containing no acid.

Example 3

In example 3, one embodiment of the scrubbing fluid comprising 39.1% byweight MDEA, 7.7% by weight piperazine, 0.47% by weight H₃PO₄, and thebalance water was used to treat a 33 MMSCFD gas stream. The gas streamentered the absorption side of the treatment at about 935 psig and about78 degrees F. The scrubbing fluid was added to the absorption side at arate of about 400-450 gallons per minute. On the desorption side,pressure was dropped to about 10.5 psig and the temperature was raisedto about 230 degrees F. at the “top” of the desorption process. At the“bottom” the inventors believe the temperature was closer to 250-255degrees F. The contaminants in the gas stream were measured before andafter scrubbing. The results are provided in FIG. 3.

Example 4

In example 4, the same embodiment of the scrubbing solution described inexample 3 was used to treat a similar gas stream at the same location asin example 3. However, in example 4, there were two inlets to theabsorption side of the process which could be independently sampled.Otherwise, the treatment conditions were the generally the same asrecited example 3. The contaminants in the gas stream were measuredbefore and after scrubbing. The results are provide in FIG. 4.

Example 5

In example 5, relatively similar gas streams were treated with differentscrubbing fluids. As shown in FIG. 5A, in tests 1 and 2 the amount of MDEA and piperazine were roughly equal, though slightly more amines wereused in test 2. In test 3, markedly fewer amines were included in thescrubbing fluid. There was also considerably less acid in the scrubbingfluid of test 3 than in test 1. The acid was added in test 1 and thennot replenished, so the results in tests 2 and tests 3 reflect adiminution in the acid content because the acid was being diluted overtime. The effect of the diminishing acid content in tests 2 and 3relative to test 1 can be seen in some of the contaminants. For example,as illustrated in FIG. 5B the methyl mercaptan exhibited a near 100%reduction in test 1 but only a 72 and 70% reduction, respectively, intests 2 and 3. Similarly, ethyl mercaptan was reduced by 99% in test 1,but only by 71 and 65% in tests 2 and 3. As seen in FIG. 5C, the amountof H₂S remaining in the scrubbing fluid after desorption increased intests 2 and 3 relative to test 1. In test 1, there was only 0.004 molH₂S remaining per mol of amine in the scrubbing fluid. In tests 2 and 3,with their lower acid content, the H₂S content remaining afterdesorption was an order of magnitude higher.

These and other modifications for the formulation of a scrubbing fluidand for the methods of using the same will be apparent to those of skillin the art from the foregoing disclosure and figures and are intended tobe encompassed by the scope and spirit of the following claims.

We claim:
 1. A scrubbing solution comprising at least one scrubbing reagent, an acid, and water, wherein said scrubbing reagent has at least one primary or secondary amine and wherein the acid is selected from the group consisting of phosphoric acids, sulfuric acids, and combinations thereof, wherein the solution has a ratio of moles of acid to moles of amine, wherein the ratio of moles of acid to moles of amine is between about 0.018 and about 0.036, wherein the scrubbing reagent is selected from the group consisting of methyl diethanolamine and piperazine and combinations thereof, wherein the methyl diethanolamine comprises between about 20 and about 60 percent by weight of the scrubbing solution, and wherein the piperazine comprises between about 0.1 and about 10 percent by weight of the scrubbing solution.
 2. A scrubbing solution according to claim 1 wherein the acid is polyprotic.
 3. A scrubbing solution according to claim 1, wherein the acid consists essentially of phosphoric acids.
 4. A scrubbing solution according to claim 1 wherein the scrubbing reagent comprises a sterically hindered amine. 